In recent years, biodegradable plastics have been actively developed to solve a problem that plastic waste has a great impact on the global environment such as impact on ecosystems, generation of harmful gas upon incineration, and global warming due to large amounts of heat generated by incineration.
Particularly, carbon dioxide emitted by incinerating plant-based biodegradable plastics was originally in the atmosphere, and therefore does not increase the amount of carbon dioxide in the atmosphere. This is called “carbon neutral”. Recently, the entry into force of the Kyoto Protocol that sets targets for reducing carbon dioxide emissions has become a real possibility. Therefore, plant-based biodegradable plastics are considered to be important and expected to be positively used.
Recently, from the viewpoint of biodegradability and carbon neutral, aliphatic polyester resins have attracted attention as plant-based plastics. Particularly, polyhydroxyalkanoate (hereinafter, sometimes referred to as PHA) resins have attracted attention. Among PHA resins, a poly(3-hydroxybutyrate) homopolymer resin (hereinafter, sometimes referred to as P3HB resin), a poly(3-hydroxybutyrate-co-3-hydroxyvalerate) copolymer resin (hereinafter, sometimes referred to as P3HB3HV resin), a poly(3-hydroxybutyrate-co-3-hydroxyhexanoate) copolymer resin (hereinafter, sometimes referred to as P3HB3HH resin), a poly(3-hydroxybutyrate-co-4-hydroxybutyrate) copolymer resin (hereinafter, sometimes referred to as P3HB4HB resin), and polylactic acid (hereinafter, sometimes referred to as PLA) have attracted attention.
However, it is known that such hard PHA resins are slowly crystallized and therefore become brittle due to a temporal change after molding processing.
Generally, a plasticizer is added to impart flexibility to a hard resin. In this case, however, there is a problem that bleed-out occurs due to the use of a large amount of plasticizer.
Patent Document 1 discloses a technique in which poly(hydroxyalkanoic acid) is blended with an ethylene/vinyl acetate copolymer so that excellent impact strength is imparted while a certain degree of transparency is maintained. According to Patent Document 1, the vinyl acetate content of the ethylene/vinyl acetate copolymer is 6 wt % or more. However, miscibility and the improvement effect vary depending on the combination of the vinyl acetate content of the ethylene/vinyl acetate copolymer and the type of poly(hydroxyalkanoic acid) used, and therefore satisfactory transparency and impact strength are not achieved.
Patent Document 2 discloses a composition obtained by blending biodegradable P3HB3HV resin with an ethylene/vinyl acetate copolymer having a vinyl acetate content of 5 to 30 wt %. According to Patent Document 2, a certain percentage of the composition has biodegradability, and the elasticity or elongation at break of the composition can be controlled within a certain range. However, P3HB3HV resin is immiscible with the copolymer, and therefore blending with the copolymer reduces transparency. Further, the ethylene copolymer needs to be blended to account for almost half of the composition in order to impart satisfactory ductility to the composition containing P3HB3HV resin, which reduces biodegradability.
Patent Document 3 discloses a composition comprising a lactic acid polymer and an ethylene/vinyl acetate copolymer having a vinyl acetate content of 30 to 90 wt %. According to Patent Document 3, the composition prevents blocking of films, has excellent elongation at break, and further has excellent transparency. However, the transparency and elongation at break of the composition are not necessarily satisfactory.
Patent Document 4 discloses a heat-shrinkable laminate film comprising: a layer comprising a resin composition containing a polylactic acid resin as a main ingredient and an ethylene-vinyl acetate resin; and a layer comprising a resin composition containing a polylactic acid resin, a polyolefin resin, and a compatibilizer that makes the polylactic acid resin and the polyolefin resin compatible. According to Patent Document 4, a heat-shrinkable laminate film having excellent transparency and ductility can be obtained by laminating at least the above-described two different layers. However, the layer comprising a resin composition containing a polylactic acid resin as a main component and an ethylene-vinyl acetate resin does not necessarily have satisfactory ductility as a film by itself.
Patent Document 1: JP-T-2010-504396
Patent Document 2: JP-T-6-503847
Patent Document 3: JP-A-9-151310
Patent Document 4: JP-A-2011-136428